Digital systems, such as memory devices, continue to operate at higher and higher speeds. Various signal lines that carry digital signals may exhibit low-pass filter (LPF) characteristics, either due to increasing channel loss with frequency, or through capacitive filtering. Thus, the maximum data rate supported by a channel becomes limited.
To compensate for LPF effects of a channel, various equalization techniques have been used. Typically, an equalizer circuit with a high pass frequency response may be provided. When a low pass channel is matched with a high pass equalizer, the overall frequency response may be flattened. One conventional approach to equalization includes modification of the signal line to make the signal line less capacitive, or modification of the signal to be less affected by capacitance, for example, by inserting repeaters or inverters on the signal line.
Another approach involves modification of the shape of a transmitted signal such that the capacitance of the signal line causes the transmitted signal to be received with a desired shape, for example, by pre-emphasis or de-emphasis. Pre-emphasis refers to increasing the amplitude of a digital signal by providing, at every bit transition, an overshoot that becomes filtered by the capacitive effects of the signal line.
De-emphasis refers to a complementary process of decreasing the amplitude of a digital signal, where at every bit transition a full rail-to-rail swing between a high supply voltage (VDDQ, VDD) and low supply voltage (VSSQ, VSS) is provided. However, in the case of de-emphasis, the full swing may overshoot the decreased amplitude of a bit-value. For example, after a transition, a first bit may have a maximum amplitude for the entire bit period. If the bit is repeated (has the same bit value), subsequent repeated bits may be de-emphasized.
One conventional way to implement de-emphasis/pre-emphasis is to utilize a delay chain to sequentially turn on or turn off the legs of a pull-up and/or pull-down circuit of a voltage driver. This causes a dynamic change in the driver output impedance, which can degrade signal integrity. Furthermore, de-emphasis/pre-emphasis is typically asymmetric, either strengthening pull-up from VSSQ or pull-down from VDDQ.
Thus, an apparatus and method for an improved signal integrity and control in bit de-emphasis is provided.